1. Field of the Invention
The present invention relates to a process for the preparation of a uretdione of 3-isocyanatomethyl-3,5,5-trimethylcyclohexyl isocyanate (isophorone diisocyanate, IPDI).
2. Description of the Related Art
The dimer of isophorone diisocyanate (abbreviated as "IPDI-uretdione" below) is an important starting compound for the preparation of polyurethane powder (PUR) hardeners which are free from blocking agents, such as described in DE 30 30 588; 30 30 539; and 30 30 572, and which are gaining appreciably in importance as a result of the ever more stringent government regulations on environmental protection.
(Cyclo)aliphatic uretdiones, inter alia also IPDI-uretdione, were described for the first time in DE-OS 16 70 720, tertiary phosphines being employed as catalysts. However, these uretdiones comprise the corresponding isocyanurates as an impurity to a considerable extent. Dimerization of IPDI in accordance with the process described in DE-OS 16 70 720 also gives no uretdione but only a mixture of reaction products which comprises IPDI-uretdione to a maximum of 80% by weight. The remainder is trimeric isophorone diisocyanate which cannot be separated off.
DE-OS 19 34 763 relates exclusively to oligomerization of IPDI with tertiary phosphines. The reaction products obtained in accordance with the disclosure of DE-OS 19 34 763 comprise about 60 parts by weight of dimeric (can be cleaved under the action of heat) and trimeric or more highly oligomerized (can no longer be cleaved under the action of heat) IPDI. By suitable process variants (for example low conversion), the dimer proportion can be increased further to about 75 parts by weight. A further increase in the uretdione content is no longer possible, since the catalyst (tertiary phosphines) catalyzes not only the dimerization but also the trimerization of IPDI to give the corresponding isocyanurate.
An isocyanurate-free IPDI uretdione is described for the first time in DE-PS 30 30 513. The catalysts employed are phosphoric acid triamides, specifically trisdimethylaminophosphine. A disadvantage of this process is that hexamethylphosphorous acid triamide, which is known to be suspected of being carcinogenic according to Br. J. Cancer 38, 418-427 (1978), becomes concentrated in the reaction product during continuous preparation of IPDI-uretdione, in which the catalyst is circulated. This disadvantage of concentration of a substance suspected of being carcinogenic in the IPDI-uretdione was to be eliminated by use of the dimerization catalysts for IPDI described in DE-OS 37 39 549 (dialkylaminopyridines substituted in the 4-position). These 4-dialkylaminopyridines, specifically 4-dimethylaminopyridine (DMAP), have become accepted for the preparation of IPDI-uretdione on an industrial scale.
The IPDI quality employed to date for dimerization of IPDI is that such as is obtained by phosgenation of isophoronediamine (DE-PS 12 02 785): ##STR2##
This standard IPDI comprises about 180 mg/kg of total chlorine, of which the hydrolyzable chlorine content is about 120 mg/kg.
DE-OSS 38 28 033 and 42 14 236 or 42 31 417 describe a process for the preparation of IPDI which dispenses with the use of highly toxic phosgene: ##STR3##
An IPDI which is chlorine-free and abbreviated as "IPDI-urea" below is obtained in this process. If the two IPDI qualities--IPDI-standard and IPDI-urea (99.6% of IPDI, as determined by gas chromatography (GC))--are compared in respect of the reaction with alcohols or of trimerization, hardly any differences are detected in these reactions. In contrast, if an attempt is made to dimerize IPDI-urea with 4-dimethylaminopyridine (DE-OS 37 39 549), highly colored reaction products, which in addition comprise small amounts of trimeric IPDI, are obtained. This phenomenon cannot be explained, since the reaction properties of IPDI should be independent of the process for its preparation. An IPDI which is unsuitable for the preparation of IPDI-uretdione would be severely limited in its possible uses.
The object of the present invention was therefore to prepare a non-discolored IPDI-uretdione from IPDI-urea.